import javax.swing.*;
import java.awt.*;
import java.awt.event.*;


/**
 * Created by ScorpionOrange on 2016/1/17.
 */
public class Calculator {
    public static void main(String[] args){
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                JFrame frame=new JFrame();
                frame.setSize(400,400);
                frame.add(new CalculatorPanel());
                frame.pack();
                frame.setTitle("Calculator");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setVisible(true);
            }
        });
    }
}

/**
 * A panel with calculator buttons and a result display
 */
class CalculatorPanel extends JPanel{
    private JButton display;
    private JPanel panel;
    private double result;
    private String lastCommand;
    private boolean start;

    public CalculatorPanel(){
        setLayout(new BorderLayout());
        result=0;
        lastCommand="=";
        start=true;
        //add the display
        display=new JButton("0");
        display.setEnabled(false);
        add(display,BorderLayout.NORTH);

        ActionListener insert=new InserAction();
        ActionListener command=new CommandAction();

        //add the buttons in a 4*4 grid
        panel=new JPanel();
        panel.setLayout(new GridLayout(4,4));

        addButton("7",insert);
        addButton("8",insert);
        addButton("9",insert);
        addButton("/",command);

        addButton("4",insert);
        addButton("5",insert);
        addButton("6",insert);
        addButton("*",command);

        addButton("1",insert);
        addButton("2",insert);
        addButton("3",insert);
        addButton("-",command);

        addButton("0",insert);
        addButton(".",insert);
        addButton("=",command);
        addButton("+",command);

        add(panel,BorderLayout.CENTER);
    }

    /**
     * Adds a buttons to the center panel.
     * @param label the button label
     * @param listener the button listener
     */
    private void addButton(String label,ActionListener listener){
        JButton button=new JButton(label);
        button.addActionListener(listener);
        panel.add(button);
    }

    /**
     * This action inserts the button action string to the end of the display text.
     */
    private class InserAction implements ActionListener{
        public void actionPerformed(ActionEvent event){
            String input=event.getActionCommand();
            if(start){
                display.setText("");
                start=false;
            }
            display.setText(display.getText()+input);
        }
    }

    /**
     * This action executes the command that the button action string denotes.
     */
    private class CommandAction implements ActionListener{
        public void actionPerformed(ActionEvent event){
            String command=event.getActionCommand();

            if(start){
                if("-".equals(command)){
                    display.setText(command);
                    start=false;
                }
                else lastCommand=command;
            }
            else {
                calculate(Double.parseDouble(display.getText()));
                lastCommand=command;
                start=true;
            }
        }
    }

    /**
     * Carries out the pending calculation.
     * @param x the value to be accumulated with the prior result.
     */
    public void calculate(double x){
        if("+".equals(lastCommand)) result +=x;
        else if("-".equals(lastCommand)) result -=x;
        else if("*".equals(lastCommand)) result *=x;
        else if("/".equals(lastCommand)) result /=x;
        else if("=".equals(lastCommand)) result =x;
        display.setText(""+result);
    }
}
